The twin twin transfusion syndrome (TTTS) is the most common serious complication of monochorionic twin gestations, affecting between 4 and 35% of monochorionic twin pregnancies each year in the United States. The TTTS accounts for 17% of all perinatal mortality associated with twin gestations. Standard therapy in the United States has been limited to serial amnioreduction which appears to improve the overall outcome but has had little impact on the more severe end of the spectrum in TTTS. In addition, survivors of TTTS treated by serial amnioreduction have an 18 to 26% incidence of significant neurologic and cardiac morbidity. Selective fetoscopic laser photocoagulation of chorioangiopagus has emerged as an alternative treatment strategy in TTTS with comparable survival to serial amnioreduction in non-randomized, non- prospective clinical studies. The superiority of fetoscopic laser treatment of TTTS remains unproven. In our preliminary data we have observed enhanced survival with selective fetoscopic laser photocoagulation in pregnancies with TTTS that had already failed serial amnioreduction or rnicroseptostomy. We hypothesize that treatment of the underlying chorioangiopagus by selective fetoscopic laser photocoagulation will not only improve the survival of twins but will reduce the incidence of neurologic, cardiac, and developmental sequelae of TTTS. We propose to test this hypothesis by a prospective randomized multicenter trial to compare serial amnioreduction with selective fetoscopic laser photocoagulation. In. the course of this study we will address the following specific aims: 1) To demonstrate that selective laser photocoagulation of chorioangiopagus in severe tETS results in significantly improved survival of twins compared to aggressive serial amnioreduction therapy. Survival of recipients and donors, both within the same pregnancy and overall survival to 30 days after birth will be examined; 2) To demonstrate that selective laser photocoagulation results in significantly improved cardiac, neurologic, and developmental outcomes in survivors of severe TTTS compared to survivors treated by aggressive serial amnioreduction. This will be a multicenter prospective randomized clinical trial with patients randomized either to serial amnioreduction at one of 16 participating centers or selective fetoscopic laser surgery at one or two participating centers. The fetal echocardiograms will be reviewed in a blinded fashion to evaluate cardiac morbidity and response to treatment.. Similarly, prenatal ultrasounds, as well as ultrasounds and MRI s obtained in the first week of life and after 4 weeks, will be reviewed in a blinded fashion to evaluate the neurologic morbidity and distinguish prenatal from perinatal events. Long-term neurodevelopmental outcome will be evaluated by the NIGH]) Neonatal Network at 18 to 22 months of age. The overall goal of the study is to improve the outcomes of twins with TTTS by determining which treatment for TTTS has a better survival as well as cardiac, neurologic, and developmental outcome.